Attic Venting In Your Home Or Business

This page has information about buildings with an attic.

 If you don't have an attic it will still relate to you how important venting of heat really is.  

The importance of venting your attic cannot be overlooked. In hot climates increase your energy costs for air conditioning and make your house uncomfortable.

Let's start with an understanding of basic physics:  Hot air rises.  But why?  Heated air rises due to a differential in density (mass), which causes an upward movement of the fluid (air is a fluid).  Like a lava lamp, the colored liquid is heated, rises to the top, cools off (density is increased), then falls back to the bottom.
Think of the force required to raise a hot air balloon off the ground with 10 passengers, the tanks of propane, the weight of the basket and the balloon.   Pretty impressive force if you think about it.   Should be easy for hot air to rise out of your attic right?  Driven by the density differential, the exhausting of the hot air should bring in fresh air where vents are located around the base of the roof (soffit). 
There is no "pressure" that drives hot air out of an attic.  The pressure (atmospheric), inside your attic is the same as it is outside (based on a no wind condition.  Wind blown across a roof can create a positive pressure on the windward side and a lower pressure on the lee side).  
Using a tracer gas and smoke generators,  we have documented all types of attic venting, the appropriate sizes that work in hot climates and have posted our findings below.   This is not a scientific study but observations made over the study of hundreds of homes in all types of climates.

Our Field Proven Results and Recommendations: (if using a radiant barrier)

•  10 square feet of venting per 1000 square feet of attic area minimum.  50% at the peak, 50% at the soffits  (with no radiant barrier, use 8 square feet per 1000 square feet of attic area in hot sunny climates)
  
• Ridge venting, placed at the peak of the roof while attractive and easy to install when replacing a roofing system, defy gravity, physics and logic.  Hot air rises, this is a law of thermodynamics yet a ridge vent consists of an inverted "V".  This creates a thermal check valve which does not allow the hot air to escape properly.
• Stay away from solar attic fans.  The claims of CFM movement are overrated.  Our testing has shown a solar fan with an 11 watt panel, (facing South), moves 315CFM.  That's less than half of what is advertised.  Good idea but not enough power to spin the fan blade.
• Whirlybirds (the aluminum spinning vents) have less than one square foot of roof opening and do not induce air movement while spinning.  Larger, less conspicuous static vents are more efficient at moving air.
• Aspirate your attic naturally (no power fans) wherever and whenever possible.  A well ventilated attic does not need power venting. 
• If you have to use a power fan (tile roofs, inaccessible areas, flat roofs), don't look for one at a home store, they are not what they used to be.  The older fans (ten years ago), had better motors, better bearings and a thermostat that could be relied upon to control the fan.   Having installed hundreds in our radiant barrier applications and replacing most of them (within a year), we were forced to find an alternative to this liability.  Our radiant barrier installations rely heavily on the removal of the waste heat generated by our products which has made us venting experts over the last 20 years. We have found a fan made in Germany used as a poultry house exhauster that is awesome:  quiet, vibration free, has permanently lubricated bearings (never needs oiling), which we mate with a quality high voltage adjustable thermostat.  (scroll down for more info).  If you have to use a fan, this is the one to use.
  

What works, what doesn't:
CONTINUOUS SOFFIT VENTING
Continuous soffit venting is typically a 2" or 3" slot cut into the soffit into which is placed a manufactured venting system made from plastic or metal.   The plastic is typically molded with thousands of small holes (1/32" in diameter), to keep bugs out.  Great idea, easy to install but falls short of offering suffient net free area to allow air to move.

Another popular type is an aluminum strip with small louvers pressed into them. 
BENEFITS:  Easy to install and attractive.
PROBLEMS:  The holes are too small (see photo below), to pass sufficient air to let the attic aspirate naturally.  Most often these are covered with insulation or partially plugged with dust/pollen/spider webs over time.
   See our formula belo

Despite the small area of these vents, the vertical stack of this vent allows for the fastest flow of air from a hot attic.  For best performance, these vents should be placed as close to the peak of the roof as possible.

RIDGE VENTING
This popular method of attic venting is catching on all over the country.   Easy to install, looks good, inexpensive, doesn't leak...only one problem:  They don't work.   Ridge vents defy gravity, physics and basic thermodynamic principles as outlined below:

• Gravity: Hot air rises.  Hot air will not turn around at the peak and go down hill, defying gravity.
• Physics:  Hot attic air is less dense than the outside ambient air and defies logic to be able to exit
  

Since our reflective attic insulation products increase attic temperature by the rejection of infrared heat, attic venting has played an important aspect of looking at the big picture.  Getting rid of this heat is very important to reduce energy consumption and increase interior comfort.   We have found through experience that homes with ridge venting are historically the hottest and emails from our customers who have read this site reinforce this over and over.
Puffing tracer gas inside the attic at 2pm just under the opening shows the air in the attic to be stagnant.   Stay away from ridge venting unless your home is built where there is a constant breeze as the air moving over the roof will tend to create a vacuum on the lee side of the roof which will draw the hot air out.

 

Dormer vents installed on a home in Dallas where the hottest attic air temperature was ever recorded by our NRG Auditor.  Two reasons:
1) The underside of the roof decking was sprayed with silver radiant barrier paint thus lowering the emissivity and increasing the convective currents (and energy use), in the attic space.
2) Ridge vents trapped the hot air.  Ridge vents rely on some unknown source of the universe to drive hot air down then out.  The dormers shown here dropped the attic temp by 42 degrees.  Solution: TCM6 over the insulation and Dormers.

SOFFIT VENTING
Many homes do have soffit vents but are blocked by improperly installed insulation.  Proper attic ventilation is necessary and the products listed below will help.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This is what you may see when you look at your venting around the perimeter of your home
"My attic has hundreds of holes around the perimeter, we don't need any more venting"

 

Remove the blocking and you can see the fiberglass blocking the vent holes.  Very common problem found during our NRG Audit inspections.

 

 

 

 

 

This is what is recommended on the outside of the home if you do not have a soffit.  If you have a soffit, add 8"x12" screened  vents every eight feet of soffit.  This procedure gives your attic the air it needs to vent properly.  In cold climates this equates to adequate moisture removal and the elimination of the associated problems like mold growth and wood rot.  In hot climates, the super heated attic air can flow out the upper vent (if you have them), and draw cooler air in around the house.  This what your architect and builder expected.  However, in most homes the insulation contractor may have plugged up some or all of your soffit vents.  Our home energy auditors have found most homes have this problem. If you have questions about your existing attic vents, take a few digital pictures and send them to us for evaluation.  This will be a free service as we fine tune our online NRG Audit coming soon.  Take advantage of our knowledge and free advice!  Please, no more than five pictures.  Take one of each side of the home noting to make photos of the soffits vents (if any), gable vents (if any), and one or two shots of the attic.

Step by Step Soffit Venting

The soffits on this home were completely blocked making the attic like an oven increasing energy costs and making the upstairs unbearable during the summer.
This is very common in homes of all ages, even brand new homes across the country.








Cutting holes in the plywood soffit to allow intake air to cool the attic. 

 


Once we get the holes cut, simply cover them with 8"x12" louvered grills.  They are available in white, silver or dark brown.





The completed installation.  Efficient, inexpensive and lasts forever.   Paint the grills after the installation to match the soffits and they blend right in.


Homes we have upgraded attic vents on have significantly reduced attic temperatures during the summer.  This means lower A/C bills, more comfort and A/C equipment lasts longer.   

A 20" x  24" dormer pictured here and a 16" round gable vent. The yellow scale pictured above is three feet long for reference.  These are the vents of choice for the best movement of air out of your attic.

This vent can be applied to 3/12 and 4/12 pitch roofs.   5/12 now available. 

 

Dormer vents installed correctly on the back side of an apartment building.  The lower vents on the roof are intake vents, the upper vents are exhaust vents.  This type of venting, if correctly sized to the attic size and volume, can adequately vent the hot air out during the summer and moisture during the winter. 
These are not seen from the front of the building and therefore do not detract from the aesthetics.

POWER VENTING

This shows a typical home store attic fan installed on a gable vent inside the attic.  Although the mounting looks secure, the application is incorrect since the space around the fan is not blocked off.   The fan will create a negative pressure behind the fan and will draw air from the closest point, in this case, from vent openings around the fan.

This means very little air will be exhausted from the attic and most of it will be drawn in around the fan.  
The open area around the fan shroud (the round ring), needs to be blocked off with cardboard or wood to make this type of installation effective.

Power Fans:
Natural attic venting is best but in many homes a powered attic ventilator is more cost effective, especially in tile roofs. 
STAY AWAY FROM UNDER POWERED SOLAR ATTIC FANS, THEY JUST DON'T MOVE ENOUGH VOLUME.  Great idea, not enough power.
 

We now import a vibration free, powerful, reliable powered attic fan manufactured in Germany.  This attic fan is unlike any other available. 
We include an adjustable thermostat and an optional mounting box with this system.  This fan features an external rotor to which the fan blades are welded.   This unique design reduces blade flex and vibration and eliminates one of the shaft bearings. 
The bearings are sealed and never need attention.  Install it, set the thermostat and forget about it. 

                  14" 1800 CFM fan shown here

Warning: In homes that have gas appliances such as water heaters or furnaces in the attic, great care must be taken to insure adequate intake venting (gables/soffits), is provided so as to not create a negative pressure that will draw combustion gasses down the chimney of gas appliances.   Installing an attic fan in this application has the potential for fire or filling your attic with carbon dioxide.  Just make certain there is sufficient intake openings to circumvent this potential hazard.      

    Construction Features

•    Motor: 1/4 Hp 115/1/60 (compare this home store fans rated at  1/8hp)
•    Adjustable thermostat included
•    Costs 2 cents per hour at 10 cents per KWH
•    CFM @ 0.0 " SP: 1839 cubic feet per minute  (Actual, not estimated)
•    Integral Blades and External Rotor allowing precision balancing
•    Formed steel wall panel with Venturi
•    Shallow profile with no protruding motor 
•    External rotor motor, class B insulation.
•   Quality design provides long and maintenance free operation for many years
•    No maintenance required
•    Automatic Reset Thermal Overload Protection
•    Electro statically applied polyester/epoxy urethane powder coat finish
•    Self lubricating, sealed for life precision ball bearings
•   Can be installed in gable vent  or under dormer vent

Specifications
Performance Data

Fan Model	RPM	Volts	Max. Watts	Max. Amps	0"	.100"	.125"	.25"	.375"	.5"	Sones**
8" PAV	1550	115	45	0.41	304	246	224	74	-	-	4.1"
10" PAV	1500	115	68	0.62	624	558	533	377	132	-	7.9
12" PAV	1400	115	130	1.19	1208	1069	1030	797	-	-	9.4
14" PAV	1200	115	245	2.24	1839	1654	1599	1295	-	-	9.6
16" PAV	1400	115	458	4.19	3054	2882	2839	2570	2198	1699	12.0
18" PAV	1550	115	698	6.39	4115	3895	3840	3549	3239	2908	14.3
20" PAV	1100	115	435	3.98	3693	3368	3279	2775	1429	-	10.7
20" PAV	1450	115*	916	8.39	4949	4682	4615	4274	3917	3445	16.4
22" PAV	1000	115*	756	6.92	5629	5248	5153	4432	-	-	12.
25" PAV	1000	115*	1134	10.38	7858	7355	7230	6567	-	-	14.8

** The sound ratings shown are loudness values in fan sones at 5ft. (1.5m) in hemispherical free
 field calculated per AMCA Standard 301. Values shown are for installation Type A: Free inlet fan
 sone levels. All sone values shown are calculated at 0.1" (static pressure in inches W.G.).

Which fan looks like it's more efficient at moving air out of your attic?  Over the years we have found home store attic fans to be a liability to our installations and the only weak link in our proven NRG Package.  Since a properly installed radiant barrier increases attic air temperature by keeping unwanted heat from getting to the insulation, removing this wasted heat has forced us to become experts in attic ventilation.
Home store fans like the one pictured are under powered, last one year (if that), and the Chinese thermostat is inaccurate, sticks on (or off), creates call backs and problems.  Sure the nice one costs more but like anything else, you get what you pay for.  



 

 

 

14" fan shown with optional fan mounting box (see below)

We fabricate this sheet metal fan box for our Phoenix installations and thought we should offer it to our VI customers as well.  
The design allows for a smoother discharge of air as it fully takes advantage of the  aerodynamic  design of  the  shroud and large side flanges makes it much easier to install.   Cuts the installation time in half.  

 

 

 

 

 

 

 

 

This is how the fan will look.   The fan is mounted to the sheet metal box and there is 15' piece of 14 gauge Romex already pre-wired to the fan and thermostat. We even provide a 2"x4" mounting box to mount the thermostat so all we need to do is run a 115v power supply to the thermostat, mount the fan shroud to the wall, set the thermostat to 105f and call it a day.